Irrational Man : A Study in Existential Philosophy
Irrational Man a Study in Existential Philosophy by William Barrett is "the" place to begin if one has never heard of Existentialism, or has a mild curiosity, and would like to get one's toes wet before diving into the extensive collection of works by the so-called Existentialist minds.
As founder of the "Realm of Existentialism", I understand how Existentialism can be quite complicated. "Barrett speaks eloquently and directly," none of that stiff, mind-boggling academic-speak that philosophy is so often written in. Barrett starts out with a history of Existentialist thought...how it came about and formed, and then ends up discussing, in mild-depth (nothing too heady), Kierkegaard, Nietzsche, Heidegger, and Sartre. The book was written in 1958, so not all the famous Existentialist minds, one has heard about (or not), are included, i.e. Camus, Kafka, Beckett, Beauvoir etc.
This is the most lucid (easily understood) definition and exploration of Existentialism I've read so far. I keep a copy of Irrational Man on my desk at all times. --Katharena Eiermann, 2005